Also push a {major} version tag (e.g. 6)

This will probably be useful during local development, now that we are
removing the `develop` tag.

.github/workflows/test-and-publish.yml

@@ -48,6 +48,7 @@ jobs:
             type=ref,event=branch
             type=semver,pattern={{version}}
             type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
 
       - name: Build and push Docker image to Docker Hub
         uses: docker/build-push-action@v5